Multicast Broadcast Service (MBS) is a popular method for wireless multimedia transmissions. The characteristic of MBS in delivering multimedia contents from a point to multi-point efficiently utilizes scarce radio resource for the wireless spectrum. Thus, MBS is widely used in the multimedia transmissions, due to the aforementioned characteristics.
On the other hand, relay is a developing technology in current wireless communication systems. By introducing intermediate relay nodes (RN) into a wireless communication system, the system capacity can be increased, the radio coverage can be enlarged, and the received signal strength can be improved. Consequently, IEEE 802.16m standard and Third Generation Partnership Project Long Term Evolution Advanced (3GPP LTE-Advanced) wireless standards adopt relay techniques.
FIG. 1 illustrates a conventional transmission scheme of a MBS system 10 with a relay node. FIG. 1 shows the basic system structure of the MBS system 10 with 2-hop relay. This conventional transmission scheme is specified by, for example, IEEE 802.16j standard. Referring to FIG. 1, the MBS system 10 includes at least a base station BS, at least a relay node RN, and several mobile stations UE1, UE2, UE3 and UE4. Between the base station BS and the base stations UE1, UE2 under the coverage of the base station BS, there is link (1). It is assumed that just broadcast or multicast delivered by the base station BS and the relay node RN, so the link with the same notation refers to the same physical signaling broadcast by the base station BW or the relay node RN. Between the base station BS and the relay node RN, there is link (2). Between the relay node RN and the base stations UE3, UE4 under the coverage of the relay node RN, there is link (3).
In IEEE 802.16j standard, link (2) is first transmitted, and then link (1) and link (3) are transmitted. The base station BS first organizes the relay nodes as a relay group, and then multicasts data to all relay nodes. After the multicast transmission of the base station BS, the base station BS and relay nodes serve the subordinate mobile stations (or user equipments) synchronously.
In the aforementioned MBS transmission scheme with the relay node, the mobile stations are served just by either the base station BS or relay nodes. In other words, the mobile stations are not simultaneously receiving data from both the base station BS and the relay nodes. If MBS is delivered in the aforementioned transmission scheme with the relay node, the transmission is inefficient as a result of the multi-hop transmission of the relay structure. Therefore, it is a major concern to modify existing MBS protocols so as to make delivery of MBS over the relay network more efficient and more reliable.